This invention relates to methods for preparing cross-linked vinylaromatic polymers containing nitro groups and having selectively with respect to cations of polyvalent and alkali metals.
Cross-linked vinylaromatic polymers containing nitro groups are used as carriers in liquid and gas-liquid chromatography, as sorbents (cation-exchangers) in chemical analysis and for separation of cations of polyvalent metals from alkali metals, for separation of mixtures of alkali metals, for purification of salt solutions of polyvalent metals from microadmixtures of other polyvalent metals, and also as catalysts in the manufacture of organic compounds.
Known in the art are methods for preparing various cross-linked vinylaromatic polymers containing nitro groups.
Common for all known methods is nitration of a polymer with a nitrating mixture containing concentrated nitric acid. Depending on the nature of the starting textured vinylaromatic polymer, the nitrating mixture consists either of concentrated nitric acid alone, or its mixture with concentrated sulphuric acid taken in various proportions.
For example, styrene divinylbenzene polymer containing phosphonic and nitro groups, is prepared in one case by nitration with a mixture of concentrated nitric and sulphuric acids and in another case by nitration with concentrated sulphuric and nitric acid, or concentrated nitric and acetic acids. The sytrene divinylbenzene polymer containing sulpho and nitro groups was prepared also in two cases by treating textured sulpho polystyrene with a mixture of concentrated nitric and sulphuric acids, and in one case by treating with concentrated nitric acid alone.
Known also is a method for preparing styrene divinylbenzene polymer containing carboxyl and nitro groups by nitration of carboxy styrene divinylbenzene cation-exchanger with concentrated nitric acid in the presence of potassium nitrate (Z.Chem. 1968, 8, (1) 26-27), and styrene divinylbenzene polymer containing only nitro groups, by treating the copolymer of styrene and divinylbenzene with a mixture of concentrated nitric and sulphuric acids (Methods for Preparing Chemical Reagents and Preparations', Moscow, IRFA, 9, 1964, 17).
The disadvantage inherent in all these methods is nitration in very strict conditions, which is especially vivid in the case with concentrated nitric acid (rather than with sulphuric acid). The result of these exacting requirements is low chemical stability of the polymers containing nitro groups, in alkali medium in particular, and also appreciable dephosphorylation, desulphonation, and decarboxylation of polymers containing functional groups during their nitration. The strict conditions make it practically impossible to obtain polymers with small amount of cross-linkages suitable for use in chromatographic columns. For example, 1 g of sulpho cation-exchanger containing 2 percent of divinylbenzene, after treatment with a mixture of concentrated nitric and sulphuric acids, occupies a volume of about 60 ml. As it is converted into the Na-form with a 4 percent NaOH solution, the specific volume is reduced to 5-6 mg/g, which makes it impossible to use the material in ion-exchange columns.
During preparation of cross-linked polymers containing phosphonic and nitro groups, and also carboxyl and nitro groups, or nitro groups alone, by the known methods, this disadvantage works and makes the polymers hardly applicable for use in ion-exchange columns. In nitration by the known methods it is impossible to prevent oxidation of phosphinic groups [--P(O)H(OH)] to phosphonic [--P(O)(OH).sub.2 ].
Furthermore, the processes utilizing concentrated nitric acid are fraught with danger of explosion and fire, and cause heavy corrosion in the process equipment.
The object of this invention is to improve the process conditions.
Another object of the invention is to rule out concentrated nitric acid from the process.